The present invention relates to an objective lens for an optical pick-up of an optical disc drive, and more particularly to an objective lens having an NA (numerical aperture) of 0.7 or more. The invention also relates to an optical pick-up employing such an objective lens.
The NA of such an objective lens is determined in accordance with a data density of a recording medium. For example, the NA of an objective lens of an optical pick-up for a CD (compact disc) is approximately 0.45. The NA of the objective lens for a DVD (digital versatile disc) is approximately 0.6.
The objective lens of the CD drives or DVD drives is generally a single element lens having aspherical surfaces as both refraction surfaces. The objective lens for the CD or DVD drive is required such that spherical aberration is well compensated for in order to converge an incident light beam as a diffraction limited spot.
Further, abaxial coma should also be compensated counting allowances for manufacturing and/or assembly errors. To meet the above requirements, the conventional objective lens, which is typically a single lens having aspherical surfaces, is designed such that the spherical aberration is compensated in a predetermined reference status (which is generally a status where parallel light is incident on the objective lens), and a sine condition is satisfied.
In the meantime, an objective lens for an optical pick-up is generally formed using a metal mold. When the metal molding is used, a clearance for moving the molding is required. Due to this clearance, between a first surface (i.e., a light source side surface) and a second surface (i.e., an optical disc side surface), a decentering (i.e., a shift between the first and second surfaces in a direction perpendicular to the optical axis) ranging from 0.001 mm through 0.004 mm occurs. Since the NA of the conventional objective lenses for the CD and/or DVD is relatively small, high order aberrations are relatively small, and since design freedom for the focal length and working distance (i.e., a distance between the surface of the objective lens closest to the disc and the surface of the cover layer of the optical disc) is relatively large, the coma due to the decentering of the surfaces can be compensated for by adjusting the surface shape of each refraction surface of the objective lens.
Recently, an optical disc having data recording density higher than that of the DVD is suggested. For such an optical disc, the NA of the objective lens is required to be 0.7 or more. However, if a focal length of the lens is shortened in order to raise the NA with remaining a predetermined working distance, the design freedom for the surface shape of the objective lens is lessened, and it becomes difficult to compensate for the decentering coma by the surface shape of the objective lens. For example, if the NA of an objective lens is 0.7 or higher, the coma generated due to the decentering of 0.004 mm greatly exceeds an allowable range, and such a lens cannot be used as the objective lens.
Japanese Patent Provisional Publication No. HEI 11-190818 discloses a high NA objective lens configured to have two lens elements. The lens disclosed in the publication well suppresses the decentering coma and the spherical aberration, and having a high NA.
However, such an objective lens consisting of two lens elements is larger in weight and volume in comparison with the objective lens having a single lens element. Therefore, for such a lens having two lens elements, a conventional fine actuator, which is designed to move the single element objective lens in its axial direction for focusing, cannot be used.
Further, the two lens elements must be fixed onto a frame and optical axes of the lens elements must be aligned with respect to each other. In such a case, the number of manufacturing processes and the number of components may increase. Further, a working distance (i.e., a distance between a rear surface of the objective lens and a surface of a cover layer of an optical disc) of the objective lens disclosed in the publication is a range of 3.5 μm through 50 μm. This working distance is significantly smaller than that of a single-element objective lens having the same focal length.